1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor. In addition, the present invention relates to a method for manufacturing the photoreceptor, and an image forming method and apparatus using the photoreceptor.
2. Discussion of the Background
Electrophotography is one of image forming methods and typically includes the following processes:                (1) charging a photoreceptor in a dark place (charging process);        (2) irradiating the charged photoreceptor with imagewise light to selectively decay the charge on a lighted area of the photoreceptor, resulting in formation of an electrostatic latent image thereon (light irradiating process);        (3) developing the electrostatic latent image with a developer including a toner mainly constituted of a colorant and a binder to form a toner image on the photoreceptor (developing process);        (4) optionally transferring the toner image on an intermediate transfer medium (first transfer process);        (5) transferring the toner image onto a receiving material such as a receiving paper ((second) transfer process);        (6) heating the toner image to fix the toner image on the receiving material (fixing process); and        (7) cleaning the surface of the photoreceptor (cleaning process).        
In such image forming methods, requisites (i.e., electrophotographic properties requisite) for the photoreceptors are as follows:                (1) to be able to be charged so as to have a proper potential in a dark place;        (2) to have a high charge retainability (i.e., to keep the charge well-in a dark place); and        (3) to rapidly decay the charge thereon upon application of light thereto (i.e., the potential of a lighted-area is low).        
Until now, photoreceptors in which one of the following photosensitive layers is formed on an electroconductive substrate have been used:                (1) layers mainly including selenium or a selenium alloy;        (2) layers in which an inorganic photoconductive material such as zinc oxide or cadmium sulfide is dispersed in a binder resin;        (3) layers using an organic photoconductive material such as azo pigments and combinations of poly-N-vinylcarbazole and trinitrofluorenone; and        (4) layers using amorphous silicon.        
Currently, organic photoreceptors using an organic photosensitive materials are widely used because of satisfying such requisites as mentioned above and having the following advantages over the other photoreceptors:                (1) manufacturing costs are relatively low;        (2) having good designing flexibility (i.e., it is easy to design a photoreceptor having a desired property); and        (3) hardly causing environmental pollution.        
As for the organic photoreceptors, the following photosensitive layers are known:                (1) a photosensitive layer including a photoconductive resin such as polyvinyl carbazole (PVK) or the like material;        (2) a charge transfer photosensitive layer including a charge transfer complex such as a combination of polyvinyl carbazole (PVK) and 2,4,7-trinitrofluorenone (TNF) or the like material;        (3) a photosensitive layer in which a pigment, such as phthalocyanine or the like, is dispersed in a binder resin; and        (4) a functionally-separated photosensitive layer including a charge generation material (hereinafter a CGM) and a charge transport material (hereinafter a CTM).        
Among these organic photoreceptors, the photoreceptors having a functionally-separated photosensitive layer especially attract attention now.
The mechanism of forming an electrostatic latent image in the functionally-separated photosensitive layer having a charge generation layer (hereinafter a CGL) and a charge transport layer (hereinafter a CTL) formed on the CGL is as follows:                (1) when the photosensitive layer is exposed to light after being charged, light passes through the light-transmissive CTL and then reaches the CGL;        (2) the CGM included in the CGL absorbs the light and generates a charge carrier such as an electron and a positive hole;        (3) the charge carrier is injected to the CTL and transported through the CTL due to the electric field formed by the charge on the photosensitive layer;        (4) the charge carrier finally reaches the surface of the photosensitive layer and neutralizes the charge thereon, resulting in formation of an electrostatic latent image.        
For such functionally-separated photoreceptors, a combination of a CTM mainly absorbing ultraviolet light and a CGM mainly absorbing visible light is effective and is typically used. Thus, functionally-separated photoreceptors satisfying the requisites as mentioned above can be prepared.
Currently, needs such as high speed recording and downsizing are growing for electrophotographic image forming apparatus. Therefore, an increasing need exists for durable photoreceptors having high reliability, which can produce good images even when repeatedly used for a long period of time while having the above-mentioned requisites.
Photoreceptors used for electrophotography receive various mechanical and chemical stresses. When a photoreceptor is abraded due to these stresses and its photosensitive layer is thinned, undesired images are produced.
In attempting to solve this abrasion problem, a technique in which a filler is included in a photoreceptor, and a technique in which a filler is dispersed in a surface of a photosensitive layer have been disclosed in Japanese Laid-Open Patent Publications Nos. (hereinafter JOPs) 1-205171, 7-333881, 8-15887, 8-123053 and 8-146641.
The photoreceptors having a surface layer including a filler dispersed in a binder resin tend to cause the following problems:
(1) Peeling of Surface Layer
When a photosensitive layer and a surface layer formed thereon have a discontinuous structure, the surface layer tends to be peeled from the photosensitive layer when the photoreceptor is repeatedly used for a long period of time.
(2) Increase of Lighted-Area Potential
When a photosensitive layer and a surface layer have a discontinuous structure, the potential of a lighted-area of the photoreceptor increases when the photoreceptor is repeatedly used for a long period of time.
(3) Poor Fine Dot Reproducibility
When a photosensitive layer and a surface layer have a discontinuous structure (i.e., the surface of the photosensitive layer is not dissolved by the surface layer coating liquid coated on the photosensitive layer), the image qualities of initial images produced by the photoreceptor are good. However, when the photoreceptor is repeatedly used, the problems mentioned in items (1) and (2) tend to occur. To the contrary, when the photosensitive layer and the surface layer have a continuous structure (i.e., the photosensitive layer is dissolved by the surface layer coating liquid coated on the photosensitive layer), the image qualities tend to deteriorate depending on the degree of dissolution of the photosensitive layer.
(4) Uneven Abrasion
When a photosensitive layer and a surface layer have a continuous structure and in addition the photosensitive layer is largely dissolved by the surface layer coating liquid including a filler and coated on the photosensitive layer, the filler is seriously unevenly dispersed at the interfacial portion between the photosensitive layer and the surface layer. When such a photoreceptor is repeatedly used for a long period of time, the photoreceptor is abraded unevenly, resulting in deterioration of image qualities.
(5) Edge Effect of Solid Image
When the surface of a photoreceptor is charged so as to have a solid latent image having a very even potential and the solid latent image is developed with a toner, the edge portion of the resultant solid toner image has a larger amount of toner particles than the other portions (this phenomenon is referred to as a so-called “edge effect”) because the electric fluxlines at the edge portion erect. Therefore, fat images and toner-scattered images are produced.
In attempting to this problem, a method in which fine uneven potentials are formed on the surface of the photoreceptor is used. By this method, the edge effect can be avoided, and therefore, the chance that fat images and toner-scattered images are produced can be decreased.
On the other hand, as the methods for forming a surface layer, spray coating methods, ring coating methods, dip coating methods, etc. are typically used.
At first, the spray coating methods will be explained.
JOP 6-308757 discloses a spray coating method using a coating liquid including a solvent not dissolving the photosensitive layer on which the coating liquid is to be coated. When coating this coating liquid using a spray coating method, the surface layer does not dissolve the photosensitive layer, namely, the photosensitive layer and a surface layer have a discontinuous structure. It is described in JOP 6-308757 that the photosensitive layer having such a structure produces images having good image qualities because the surface layer coating liquid does not dissolve the photosensitive layer.
When this photoreceptor is prepared by the present inventors according to the method described in the publication, the photosensitive layer and a surface layer have a discontinuous structure. When image qualities of such a photoreceptor are evaluated, initial images have good image qualities but the surface layer peels from the photosensitive layer at the edge portion of the photoreceptor when the photoreceptor is repeatedly used. This is because the surface layer has poor adhesion with the photosensitive layer. In addition, when the photoreceptor is repeatedly used, the lighted-area potential increases and thereby image qualities deteriorate. This is because the charge injection from the lower layer (photosensitive layer) to the upper layer (surface layer) is obstructed due to the discontinuous structure of the surface layer and the photosensitive layer. In addition, it is possible that by using a surface layer coating liquid including a solvent not dissolving the photosensitive layer, the charge transport material in the photosensitive layer tends to crystallizes, and thereby undesired images are produced.
JOP 6-89036 discloses a spray coating method using a coating liquid including a solvent dissolving the photosensitive layer on which the coating liquid is to be coated. When such a coating liquid is coated using a spray coating method, the solvent dissolves the binder resin in the photosensitive layer, and thereby the surface layer is mixed with the photosensitive layer at their interface. Therefore, the photosensitive layer and the surface layer have a continuous structure. When such a photoreceptor is repeatedly used, such a peeling problem as mentioned above does not occur because the surface layer has good adhesion with the photosensitive layer. However, since the mixing conditions of the layers are not specified, other properties (such as image qualities) of the photoreceptor are not necessarily good because the properties largely change depending on the mixing conditions.
Then the ring coating methods will be explained.
JOP 8-292585 discloses a method in which a surface layer is formed by coating a coating liquid including a solvent dissolving the photosensitive layer using a ring coating method. When such a coating liquid is coated using a ring coating method, the solvent dissolves the binder resin in the photosensitive layer, and thereby the surface layer is mixed with the photosensitive layer at their interface. Namely, the photosensitive layer and the surface layer have a continuous structure. When such a photoreceptor is repeatedly used to evaluate the image qualities, such a peeling problem as mentioned above does not occur and in addition the lighted-area potential hardly increases. However, the image qualities are not good. This is because the conditions of the surface layer and the coating conditions are such that the resin and other components included in the photosensitive layer are excessively dissolved into the surface layer.
JOP 5-722749 discloses an image bearing member in which a surface layer coating liquid including an electroconductive particulate material and a solvent dissolving the lower layer (i.e., heat-softening layer) on which the coating liquid is to be coated is coated on the lower layer. However, there are no descriptions with respect to the coating conditions, and in addition the mixing conditions of the surface layer and the lower layer are not described. Therefore it is unknown whether the properties of the resultant image bearing member are good.
Because of these reasons, a need exists for a photoreceptor which has good mechanical durability and stable electrophotographic properties such that images having good image qualities can be stably produced even when the photoreceptor is repeatedly used for a long period of time.